The aim of this proposal is to determine the functional relationships among gap junction coupling, neuronal specification and synaptic connectivity in the developing CNS. These issues will be explored in mouse spinal cord and neuromuscular synapses. Motor neuron coupling has been well studied; however, many questions remain about the role of coupling at early times in development, and the hypothesis that coupling modulates synaptic connectivity in skeletal muscles has not been tested. I have developed a novel method of labeling coupled cells at developmental stages when cell injection techniques are difficult. By determining the spatial and temporal pattern of coupling among motor neurons through development, I will test the hypothesis that gap junctional coupling modulates motor neuron differentiation and motor pool formation. By generating a transgenic mouse over-expressing a gap junction protein in motor neurons, I wilt test the hypothesis that motor neuron coupling affects the formation and elimination of synaptic connections with skeletal muscle targets. These experiments will extend our understanding of the functional roles of neuronal coupling in shaping the patterning and synaptic organization of the nervous system.